1. Technical Field
The present invention relates to a unit for monitoring the state of an assembled battery which includes a plurality of unit cells connected in series in order to output a desired voltage.
2. Background Art
For example, a hybrid vehicle includes an assembled battery which acts as a motor driving power source. Such an assembled battery obtains a high voltage by connecting, in series, a plurality of unit cells of a secondary battery (storage battery) such as, for example, a nickel-hydrogen battery, a lithium battery or the like.
For such an assembled battery, there is a need to monitor a charging state of each unit cell in order to prevent each unit cell from being over-discharged or overcharged. The monitoring of the charging state of each unit cell requires the measurement of a voltage of each unit cell which is an indicator of the charging state.
Typically, a control system for an assembled battery includes a plurality of high voltage side controllers connected respectively to charging/discharging circuits of the assembled battery and a low voltage side controller electrically isolated from the plurality of high voltage side controllers. Each of the plurality of high voltage side controllers is provided to correspond to each unit cell and measures a voltage of each unit cell. The measured voltage of each unit cell is output to the low voltage side controller via a non-contact connector such as a photo coupler or the like. The low voltage side controller commands each high voltage side controller to measure the voltage of the unit cell and receives a measurement voltage of the unit cell measured by each high voltage side controller in response to the command. The measurement voltage of the unit cell received from each high voltage side controller is output, either as it is or after being subjected to a process, to a high level computer equipped within the vehicle.
As communication between the low voltage side controller and each high voltage side controller, a daisy chain scheme is mainly used which can easily cope with a change in the number of high voltage side controllers and has a high scalability. Taking the above-mentioned communication between the low voltage side controller and each high voltage side controller as an example, in the daisy chain communication, a protocol of a so-called master-slave scheme is commonly used in which the low voltage side controller corresponds to a master device and each high voltage side controller corresponds to a slave device (for example, see JP-A-2003-196230).
However, when data communication is performed between the low voltage side controller and each high voltage side controller, the amount of communication data increases with the increase of the number of high voltage side controllers, which is in proportion to the number of unit cells. In particular, in the communication using the above-mentioned master-slave scheme, since round-trip data communication is individually performed between the master device and each slave device, the amount of data communication becomes more remarkable when the number of high voltage side controllers increases.
The amount of data communicable per unit time between the low voltage side controller and each high voltage side controller is defined by the specification of a corresponding communication line. If the communication line used is a high speed communication line, a disadvantageous factor such as relative deterioration of noise resistance occurs and accordingly there is a desire to exchange measurement voltage data of unit cells between each high voltage controller and the low voltage controller without using such a high speed communication line.
As one measure to meet such a desire, it may be contemplated that the low voltage side controller issues a common command to instruct all of the high voltage side controllers to measure voltages of the unit cells. However, when each high voltage side controller outputs measurement voltage data of the unit cells to the low voltage side controller in response to the common command from the low voltage side controller, there is a need for the low voltage side controller to accurately detect that the measurement voltage data come out of the plurality of high voltage side controllers.